Several diversity transmission techniques have been developed in the past both for the downlink (DL) and for the uplink (UL) of OFDM systems. Actually the most well-known ones are:    1. The switched diversity technique, which requires a single transmitter (TX) chain in the UL and therefore only one power amplifier. This technique offers good performance in slowly varying environments in which deciding in favor of one antenna among two makes sense. On the other hand, in fast varying environments, it is of no interest since the decision of the best antenna that is done on the DL signal is no longer valid when it is time to send the UL signal.    2. The “two transmission tile allocation”, which requires two transmission chains but is transparent to the receiver. This technique consists in sending each group of subcarriers (called tiles in Wimax) from the best Tx antenna (the choice is based on the observation of the channel obtained in the DL in order to optimize performance) and an exact attribution of the tiles to the best antenna, which is a quite long operation. This kind of techniques that consist in distributing the subcarriers on several Tx antennas actually is a well-known technique that appears in many other patents.    3. The “Cyclic Delay Diversity (CDD)”, which requires two transmission chains but is transparent to the receiver. This technique is extensively used in Wimax in the downlink channel.    4. The Alamouti scheme (also known as STC) which also requires two Tx chains and offers improved performance compared to CDD but requires a special decoder to be implemented at the receiver side.
In addition to the need for one or two transmission chains and to the need for a special decoder at the receiver side, another important criterion to compare different systems is the maximum transmit power available on the single transmission chain or on each of the two transmission chains. In the following, we use the triplet (Diversity scheme, PTX1, PTX2) to represent a system, where PTX1 (resp. PTX2) denotes the maximum transmit power of the first (resp. second) transmission chain.
Actually, when comparing the performance of transmission schemes in a real context (WiMAX system with convolutional coding and real channel), it turns out that the performance of (Switched diversity, 2P, 0) significantly outperforms the performance of (CDD, P, P) and (STC, P, P) in slowly varying environment in which the antenna choice can be done reliably.
On the other hand, (CDD, 2P, 2P) and (STC, 2P, 2P) clearly work better than (Switched, 2P, 0) but is much more costly.
Then (two transmission tile allocation, P, P) outperforms (Switched diversity, 2P, 0) in most of the cases except when the antenna imbalance is high.
When antenna imbalance is higher than 8 dB, (CDD, P, P) is better than (two transmission tile allocation, P, P) and almost equivalent to (Switched diversity, P, 0).
The general concept of two transmission tile allocation is already known in the literature as “clustered OFDM”. In the case of two antennas, it consists in sending the first half of the subcarriers on the first antenna and the second half on a second antenna according to a fixed pattern (typically the transmit antenna is changing every X subcarriers).
Instead of applying a fixed and deterministic pattern, a very general and well known technique consists in choosing the transmission antenna every X subcarriers based on the knowledge of the channel acquired in the DL of a TDD system. Such a technique can be found US patent application 2005/0113041 called “Frequency domain subchannel transmit antenna selection and power pouring for multi-antenna transmission”.